The present invention relates to the fabrication of films, particularly to a low cost method for making thick or thin films, and more particularly to a method of making films wherein a prepared solution is deposited on a substrate to be coated using an atomizer to produce films of various thickness and with flexibility.
A number of low cost non-vacuum based thick/thin ceramic film casting techniques are readily available. These include tape casting, tape calendering, colloidal dip coating and spin coating, screen-printing, gel casting, slip casting, pressure casting, etc. All of the above techniques require somewhat similar preparations of the slips containing the ceramic powder with different additives and solvents. However, the application approach of the slip differs from one technique to another, making certain techniques more suitable for certain specific applications. For instance, colloidal dip coating consists of dipping the substrate in the colloidal solution followed by withdrawing the substrate at constant speed. After complete evaporation of the solvent, a film is formed. There is little active control of the film thickness, the process must be done using trials and errors approach to find the right viscosity and the right solid loading in the solution in order to obtain the right film thickness. This technique is not limited by the size or shape of the substrate but there is a limit in the maximum film thickness (typically 10 xcexcm) beyond which film cracking occurs because of drying shrinkage.
Tape casting requires the preparation of a more viscous slip and more additives such as binders, plasticizers. The slip is spread on a flat surface to a controlled thickness with the knife edge of a doctor blade, and the solvents are allowed to dry. The resulting tape, containing ceramic powders, plasticizers and binders is flexible. It can then be stripped from the casting surface, cut to size, then fired at high temperatures. Multilayer tapes are fabricated by sequential casting one layer on top of another. This technique is largely used in a number of areas including the microelectronic industry and the energy industry. Due to the use of the doctor blade, tape casting is limited to flat samples only. Also due to the use of the doctor blade that need to be kept parallel but separated from the substrate at a constant distance, tape casting is typically limited to films of thickness greater than say 50 xcexcm. Thinner films can be prepared, however, this would require that the substrate must be very flat, which is difficult to achieve in certain applications. For very large size samples, say several feet long, such a flatness is almost impossible.
The present invention is a simple technique that can replace and/or complement the conventional tape casting technique while providing more flexibility with simplicity. The invention involves a method using a solution or slip similar to the prior techniques but with higher volatility and lower viscosity. The method involves depositing the solution via an atomizer and upon hitting a desired substrate the solution solvent evaporates, leaving a green tape or film containing a desired powder and other additives. The spraying can be carried out at room temperature and above, and deposited on a slightly heated substrate to accelerate the evaporation of the solvent.
It is an object of the present invention to provide a method for producing films of various thickness.
A further object of the invention is to provide a film forming method using an atomizer.
A further object of the invention is to provide a method for providing flexible films of about 1 xcexcm and thicker.
A further object of the invention is to provide a method for producing flexible or non-flexible film on surfaces of various configurations.
Another object of the invention is to provide a method for producing films that can replace and/or complement the conventional tape casting technique while provide more flexibility with simplicity.
Another object of the invention is to provide a method for producing films wherein the solution contains a powder to be coated dispersed in a volatile solvent such as ethanol, and containing other desire ingredients, and wherein the solution is sprayed on a substrate using an atomizer, and upon hitting the substrate the solvent evaporates.
Another object of the invention is to provide an aerosol spray casting (ASC) technique for producing green flexible films, which can be punched, cut and heated.
Another object of the invention is to provide an ASC technique which can be carried out at room temperature or at elevated temperatures below the evaporation temperature of the solvent used, and which can be carried out using heated substrates.
Other objects and advantages of the present invention will become apparent from the following description. The invention is directed to a method for making thin and thick films at very low cost and which films are more flexible and versatile than films produced by the conventional tape casting technique, for example. The method involves ASC and produces a green flexible film, similar to the tape casted film but with several advantages:
1. The method is not limited to flat surfaces but can be considered for use on substrates of any size and shape, such as thin film casting on tubes.
2. The method can be used to deposit much thinner films than conventional tape casting because there is no requirement of exact distance between the spray nozzle (atomizer) and the substrate. The method can be carried out at room temperature or at an elevated temperature with or without the substrate being heated. The composition of the slip or solution used may be similar to that of the conventional tape casting slip, but includes a dispersant to prevent agglomeration, and the solvent has a higher volatility and the solution or slip has a low viscosity, whereby, solution is deposited as a fine mist, and the solvent evaporates upon hitting the substrate. The method can be used in a variety of applications requiring thin or thick films, such as in the fabrication of solid oxide fuel cells, ceramic membranes, multilayer films, etc.